Implantable medical devices are often configured to be used in conjunction with an external programmer that allows a physician to display information detected by the implanted device. For example, the external programmer may operate to display electrical cardiac signals detected by the implantable device in the form of IEGMs. An IEGM is a graphic depiction of electrical signals emitted by active cardiac tissue as detected by electrodes placed in the heart. The electrical signals are digitized and recorded with the implanted device along with an indication of the date and time, and ultimately transmitted to the external programmer for display thereon, typically during follow-up sessions with the physician sometime after the device has been implanted. Alternatively, the implanted device is controlled to transmit the IEGM signals in real-time during the follow-up session. The external programmer may also be configured to receive real-time surface electrocardiogram (ECG) signals from an external ECG detector, for display along with contemporaneous real-time IEGM signals transmitted from the implanted device.
The implanted device may also be configured to detect various events, such as paced and sensed events or premature atrial contractions (PACs), and to generate event codes representative of the events for transmission to the external programmer. The events are detected and event codes are stored in the implanted device along with the corresponding IEGM signals for subsequent transmission to the external programmer or are detected and transmitted to the external programmer in real-time along with contemporaneous IEGM signals. The external programmer generates event marker icons based on the event code and displays the icons along with the IEGM signals and surface ECG signals. Exemplary event markers are: “P” for a sensed event in the atria; “R” for a sensed event in the ventricles; “A” for a paced event in the atria, and “V” for a paced event in the ventricles. Along with the event markers, the programmer may also display variable length horizontal lines representative of the length of atrial and ventricular refractory periods associated with certain events, as well as numerical values indicative of heart rate and indicative of various measured intervals between atrial and ventricular events, based on still further information recorded and transmitted by the implantable device.
Such displays of IEGMs and event markers are helpful in permitting the physician to diagnose arrhythmias and to program the implanted device to provide optimal therapy. U.S. Pat. No. 5,431,691 to Snell et al. entitled “Method and System for Recording and Displaying a Sequential Series of Pacing Events” provides a description of the operation of an exemplary pacemaker and external programmer including a detailed description of the generation, transmission and display of IEGM data and event markers. U.S. Pat. No. 5,431,691 to Snell et al. is incorporated by reference herein.
Given the memory space limitations and power limitations within implantable medical devices, it is desirable to detect and record IEGMs and corresponding event markers only during periods of interest, such as during an arrhythmia. Accordingly, some implantable devices are configured to begin recording IEGM and other diagnostic data only upon detection of an actual arrhythmia. Although this technique is helpful in reducing power and memory consumption, by activating the recording of diagnostic data only upon detection of arrhythmia, the device therefore does not record diagnostic data leading to the onset of the arrhythmia, which is often of particular interest to the physician. In this regard, it is often desirable to program the implanted device so as to deliver therapy to prevent the onset of an arrhythmia, rather than to merely respond to an arrhythmia once one has already occurred. IEGM and other diagnostic data recorded prior to the onset of arrhythmia is very helpful as it allows the physician to review the circumstances leading to the arrhythmia so that preventative therapies may be adjusted as needed to achieve optimal effectiveness.
To remedy this problem, certain implantable devices are configured to allow so-called “pre-trigger data” to be saved along with data recorded during an arrhythmia. Briefly, the device continuously detects and records diagnostic data in a circular first-in/first-out (FIFO) queue. If an arrhythmia is detected, the diagnostic data recorded just prior to the onset of the arrhythmia is transferred from the FIFO queue to long-term memory, so that it can be saved along with data recorded during the actual arrhythmia for subsequent review by the physician. In this manner, diagnostic data detected during the period of time leading to the onset of the arrhythmia is saved in long-term memory for subsequent review by the physician, without requiring that all diagnostic data be saved in long-term memory at all times. Hence, the term “pre-trigger data” refers to diagnostic data that is temporarily recorded prior to the detection of an event triggering long-term recordation of diagnostic data. An example of a pre-trigger technique and is set forth in U.S. Pat. No. 5,732,708 to Nau et al., entitled “Method for Storing EGM and Diagnostic Data in a Read/Write Memory of an Implantable Cardiac Therapy Device.”
Although pre-trigger recording techniques represent a significant improvement over techniques that fail to save data occurring prior to the onset of an arrhythmia, disadvantages remain. In particular, pre-trigger techniques require that the pre-trigger memory be active at all times, which results in a considerable drain on the power supply of the implanted device. Indeed, for state-of-the-art devices, the need to continuously record diagnostic data in pre-trigger memory can result in the loss of six months of device longevity. Accordingly, it would be highly desirable to provide techniques that allow for saving pre-arrhythmia or other pre-trigger diagnostic data for subsequent review but which do not require that the pre-trigger memory be continuously active. It is to this end that the present invention is primarily directed.